FIG. 26 shows a schematic view of a conventional document-size detection system using a detection method based on the amount of reflected light.
Conventionally, a sensor which detects the amount of reflected light has been used as a document size sensor for use in the document reader of copying machines. In the sensor of this type, which uses the detection method based on the amount of reflected light, a light-emitting element 81 such as an infrared light-emitting diode (LED) and a light-receiving element 82 such as a phototransistor are installed below a transparent glass plate 83 (document platen) whereon an original 84 is placed.
When there is no original 84 on the glass plate 83, a light beam emitted from the light-emitting element 81 is separated into two beams, one reflected off the glass plate 83 and the other passing through the glass plate 83. Only the beam reflected off the glass plate 83 is incident to the light-receiving element 82. In this case, the amount of light that is incident to the light-receiving element 82 is comparatively small.
In contrast, when there is an original 84 on the glass plate 83, the beam that passes through the glass plate 83 is reflected off the original 84; therefore, both of the beams reflected off the glass plate 83 and reflected off the original 84 are incident to the light-receiving element 82. In this case, the amount of light that enters the light-receiving element 82 is comparatively large.
As described above, when the amount of reflected light is large, detection is made with the result that "the original 84, as a reflective object, is present" at the sensing position of the sensor. When the amount of reflected light is small, detection is made with the result that "the original 84 is absent" at the sensing position of the sensor. Conventionally, a plurality of such sensors detecting the amount of reflected light are installed, and by detecting the presence and absence of the original 84 at the respective sensing positions, the size of the original 84 is detected.
Therefore, when the original 84 that is placed on the document platen 83 is whitish, that is, when the color of the document 84 has a high reflection factor, the amount of reflected light received by the light-receiving element 82 increases to a great degree, thereby providing an easy detection.
However, in the case when the original 84 is dark-colored like that obtained by copying a photograph, that is, when the color of the document 84 has a low reflection factor, the amount of reflected light received by the light-receiving element 82 does not increase so much. For this reason, the sensor tends to make an erroneous detection that "the original 84 is absent" in spite of the fact that the original 84 is present.
Further, in the event of deterioration of the LED due to long-time use or an insufficient amount of light in the LED due to changes in ambient temperature, the erroneous detection of the size of the original 84 might occur more frequently with respect to a dark-colored original 84. In these cases, even if an original 84 with a reflection factor that is not so low is used, the erroneous detection of the size of the original 84 might occur.
Moreover, as illustrated in FIG. 27, in the conventionally used sensor wherein the detection method based on the amount of light is employed, the same number of sensors (six in FIG. 27) as that of types of document sizes to be detected and identified is required. (Here, the types of document sizes are six: A3, A4, A4R , B4, B5, and B5R according to Japanese Industrial Standard [JIS].) For example, if number (1) through number (4) sensors detect "the presence of the original" while number (5) and (6) sensors detect "the absence of the original", the detection is made with the result that "the size of the original is A4R". This arrangement has required a large number of sensors, that is, a large number of the light-emitting elements 81 and the light-receiving elements 82, depending on the number of sensing positions. Such a large number of units result in an increase in cost and complicated installation work, and those members such as fixing members are also required for each unit, thereby causing adverse effects in space saving of the document reader.